1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to a pneumatic tire having considerably improved durability, in which wear resistance is durable in practical use without damaging the cornering performances and heat durable performance in summer season as well as spring and autumn seasons (hereinafter referred to as summer season). Also ice and snow performances such as driving performance, braking performance, cornering performance and the like on ice and snow road surface in winter season are improved and further dynamic compression permanent deformation (so-called flattening resistance) and separation resistance are improved.
2. Related Art Statement
In conventional pneumatic tires, in order to ensure driving performance, braking performance and cornering performance during the running on ice and snow road surface (hereinafter referred to as ice-snow performances, simply), a chain is attached to the tire. Alternatively a spike tire is obtained by striking spiked pins into a surface portion of a tire tread and is frequently used. However, microparticles produced by wearing of the chain and spike pins or the wearing of the road are scattered to cause dust nuisance. Also road damage is caused by the spike pins, which comes into a large social problem. For this end, a so-called studless tire using no spike pin has been recently proposed, and various investigations on the tread pattern and rubber material for such a studless tire are made. However, the ice-snow performances of the studless tire are not yet developed up to a level equal to those of the spike tire. Particularly, there is an attempt that in order to ensure rubbery elasticity at a low temperature, a polymer having a low glass transition point is used as the rubber material for the tread. An attempt has been made that a softening agent having a low melting point is used for ensuring the friction coefficient on road surface at a low temperature, and the like, but these attempts are still insufficient in ice-snow performances.
On the other hand, tires using an expanded rubber in the tread are proposed in Japanese Patent Application Publication No. 40-4,641, Japanese Patent laid open No. 56-154,304 and U.S. Pat. No. 4,249,588. In Japanese Patent Application Publication No. 40-4,641, however, a synthetic rubber having a large hysteresis loss such as high-styrene rubber is used in the tread, which raises the glass transition point of rubber to increase the hardness of rubber at a low temperature, so that it is unfavorable in view of the ice-snow performances.
Further, Japanese Patent laid open No. 56-154,304 discloses a light weight tire by using an expanded rubber so as to provide a hardness equal to that of nonexpanded rubber, which can not improve the ice-snow performances.
Moreover, when the tire using the expanded rubber in the tread is used over a long period on dry road surface and ice and snow road surface, the wearing of the tread rubber is fast and the wear resistance is not sufficient. In this connection, the improvement of the expanded rubber for solving the above problem has previously been proposed in Japanese Patent Application No. 61-235,921, but it has been confirmed that the flattening resistance is not yet sufficient in practical use.